Most of the world's iron is made in blast furnaces. A blast furnace is an example of a vertical shaft furnace. The primary function of a blast furnace is to reduce iron ore to iron. A charge comprising iron ore, coke and fluxing ingredients is introduced into the top of the blast furnace to form a bed. A blast of pre-heated air is used to burn the coke to form carbon monoxide. The carbon monoxide reduces the iron ore to iron.
The heat liberated by the combustion of the coke is used to melt the iron, Iron and slag are removed as molten products at the bottom of the furnace. The air blast is introduced near the bottom of the furnace so as to form a high temperature oxidation region beneath the bed. The blast may be enriched in oxygen in order to facilitate the production of high temperatures typically in the range of 2100.degree. to 2200.degree. C., in the furnace raceway.
Such processes are disadvantageous because they consume large amounts of coke. One way of achieving a reduction in coke consumption is to inject particulate coal into the furnace with the air blast. An apparatus for performing this step is described in U.S. Pat. No. 4,887,800. One disadvantage associated with the injection of coal into the furnace is that, relative to coke, coal requires a greater proportion of oxygen for combustion and creates a greater thermal load on the blast furnace. Accordingly, the greater the amount of coal injected into the blast furnace, the larger the amount of oxygen in the blast air needed to maintain the temperature of the raceway in the chosen range of 2100.degree. to 2200.degree. C. This disadvantage has restricted the use of coal in the manufacture of iron in a blast furnace and other metallurgical processes.
Various processes are known for pretreating coal so as to improve its combustion. For example, U.S. Pat. No. 3,250,016 discloses a method of preparing coal for injection into the tuyeres of a blast furnace. The method comprises feeding coal fines and top gas from the blast furnace to a fluidized bed drier, discharging the dried coal from the drier, screen-sizing the dried coal to separate undersized particles having a maximum size of about 3.2 mm that are suitable for injection, pulverizing the oversized coal particles for further screen-sizing, and cleaning, compressing, and drying the off-gas from the drier. This method involves drying the coal at relatively low temperatures in a reducing atmosphere. European Patent Application No. 467,375 discloses using a non-oxidizing atmosphere, for example, nitrogen to dry the coal. Neither U.S. Pat. No. 3,250,016 nor European Patent Application No. 467,375 discloses the step of driving off volatile hydrocarbons from the coal in a pretreatment region. Accordingly, neither of these references provide a means of overcoming the above mentioned disadvantage.